1. Field of the Invention
The present invention relates to an automatic focus control device for use in a camera system, more particularly, to an automatic focus control device for use in a camera system for detecting a focus condition and controlling an automatic focus adjustment in accordance with an output of an image sensor.
2. Description of Prior Art
There is provided a camera system comprising an automatic focus control device for detecting a focus condition such as the amount of defocus and the direction of defocus in accordance with a reflected light from an object to be photographed, and controlling an automatic focus adjustment in accordance with a result of the focus condition detection. In this sort of automatic focus control device, it is possible to detect an infocus position correctly, when the amount of defocus or a defocus value, corresponding to a difference quantity between a current position and an infocus position of a photograph lens of the camera, is only within an available range of the focus detection. On the other hand, it is impossible to detect the infocus position when the defocus value is out of the detection available range, in this case, a low contrast search control is performed as follows. That is, the focus condition is detected moving the photograph lens into another position, and the infocus position of the object is detected when the photograph lens is positioned in the detection available range. The low contrast search control is disclosed in the Japanese patent laid open No. 45212/1986.
However, in the aforementioned low contrast search control, when a cycle period of the focus detection is longer than a defocus variation during the photograph lens's moving and the moving quantity of the photograph lens during the lens's movement becomes larger than a predetermined detection available range, this leads to a problem that a lack of detecting an infocus position of the object by overrunning the infocus position during the lens's movement is caused and it is impossible to detect the focus position.
The reason why the lack of the infocus position detection for the object occurs will be described below in details, referring to FIG. 26. The left extreme position of FIG. 26 corresponds to the nearest photograph position of the camera to the object, on the other hand, the right extreme position corresponds to the infinity position, and x denotes the position of the object. Now, the photograph lens LENS1 is positioned at a position SA-1, and DFC denotes a range where the focus condition, i.e., the amount of defocus and the direction of defocus can be detected, wherein the range is referred to hereinafter as detection available range. Then, since the object is far from the detection available range of the photograph lens, the focus condition is detected moving the photograph lens in the infinity direction. Within a required time for detecting the focus condition at the point SA-1, the photograph lens is moved at a distance DFMA into a point SA-2 and the second focus detection is performed at the point SA-2. If the required time for detecting the focus condition is constant, the focus detection is performed in order at the points SA-3 and SA-4, and the infocus position of the object can be detected when the lens LENS1 is positioned within the detection available range DFC of the photograph lens LENS 1. As described above, when the moving quantity DFMA of the photograph lens LENS 1 during one unit time for the focus detection is smaller than the detection available range DFC, the focus detection can be performed.
On the other hand, when the moving quantity DFMA of the photograph lens during one unit time for the focus detection is larger than the detection available range DFC, the lack of the infocus position detection may occur. That is, when the photograph lens LENS 2 is moved from the point SB-1 in the infinity direction during the low contrast search, the focus detection is respectively performed in order at the points SB-1, SB-2, SB-3, and SB-4. However, when the photograph lens LENS 2 is positioned at the point SB-5, the infocus position detection can not be performed because the amount of defocus between the position of the photograph lens LENS 2 and the infocus position, i.e., the position of the object x is larger than the detection available range DFC. Next, when the photograph lens LENS 2 is moved by the moving quantity DFMB, the photograph lens overruns the infocus position x and is positioned at a point SB-6. In this case, the infocus position detection also can not be performed because the amount of defocus is larger than the detection available range DFC, as well as the detection at the point SB-5. Thus, when the moving quantity DFMB of the photograph lens during one unit time for the focus detection is larger than the aforementioned detection available range DFC, the lack of the infocus position may occur, resulting in that the infocus position can not be detected.
Furthermore, not only the focal point can not be detected, but also the following problem may often occur even though the image of the object may be brought into an infocus condition. That is, when the aforementioned low contrast search is performed, the photograph lens is moved from an out of focus position to an infocus position, and overruns the infocus position without stopping, resulting in that the photograph lens is positioned at another out-of-focus position. Therefore, the reliability of the automatic focus control device is reduced.